A combined service and parking brake system of this general type is disclosed in German patent application No. 195 02 927.5.
In this case, application of the brake in both modes of operation (service mode and parking mode) is effected hydraulically, and the brake is locked in a parking brake operation with a pressure impact which exceeds the operating pressure. The release operation of the parking brake also necessitates a pressure impact which is produced by an independent energy source. These pressure impacts above the operating brake pressure range impair the ventilating function of the sealing ring of the brake piston. Due to the high pressure the brake caliper is expanded to such an extent that a relative movement occurs during brake application between the sealing ring and the brake piston. This slip renders it impossible to the sealing ring to return the brake piston into it original position in the unloaded and unlocked condition of the brake. Disadvantageous residual torques are caused. Another disadvantage of locking and releasing the parking brake by pressure impacts which considerably exceed the operating brake pressure range, is that the brake is additionally damaged by the pressure impacts. The more the inclination of the Woehler-line flattens the worse the additional damage becomes. The result is that fatigue strength is not achieved with the reliability desired, or that the brake must be greatly reinforced (increase in weight).
Also, combined service and parking brakes are known wherein there is provision of a hydraulic actuating device for service brake operations and mechanically actuated locking devices as a parking brake. However, these arrangements suffer from the shortcoming that an additional brake cable must be provided beside the hydraulic line for each brake. This increases the expenditure in material and manufacture. Further, the brake cable cannot be mounted until the installation of the brake into the vehicle. Usually, such an assembly is not made by a particularly competent brake manufacturer but is generally carried out by car makers. Beside the disadvantage of increased assembly efforts, car makers additionally bear the risk of wrong assembly.
German published patent application No. 42 05 590 discloses a brake system where service brake operations are carried out hydraulically and parking brake operations are assisted by an electromotive adjustment means. Such an electromotive adjustment means, which takes direct effect on the position of the brake pedal, relieves the driver from the load of force application during a parking brake operation, but involves additional structural effort. This also causes additional costs.
An object of the present invention is to improve upon the prior art service and parking brake systems such that the parking brake function can be achieved easily and inexpensively while satisfying the requirements mandated by law.
According to the present invention, this object is achieved wherein the hydraulically applied wheel brake is lockable in opposition to the clamping direction of the brake by a locking device which is operable electrically. In a preferred aspect of the present invention, the mechanical locking device which is adapted to be deactivated electrically is triggered by pressure build-up in the combined service and parking brake system, with the result that the wheel brake is locked in its clamped position. For such an embodiment of the service and parking brake system, the electric deactivation of the mechanical locking device can be controlled in a service brake operation by way of the position of the brake light switch. Deactivation of the locking mechanism during service brake operations is advantageously maintained for a time after the actuation of the service brake to ensure that the wheel brakes are totally released.
In a preferred and simple aspect of the present invention, the locking device is a mechanical catch mechanism which acts unilaterally, permits movement in the clamping direction of the wheel brake and prevents movement in the opposite direction.
A catch mechanism of this type may have a form-lock design, in the form of a rail with a saw-tooth snap-in configuration and an associated pawl, or may be a frictional means in the form of two rails, movable parallel to each other, with a wedge-shaped interspace and a stop element in this wedge-shaped interspace which produces a clamping effect when the rails move in the locking direction.
In order to satisfy legal requirements demanding a second actuating travel for the parking brake, the parking brake is hydraulically actuated either by muscle power of the driver by way of a braking pressure generator (for example, second brake pedal, lever) which is independent of the operating system and adapted to be operated by hand or foot, or by an independent energy actuating mechanism. It is necessary for both types of actuation that the parking brake can be released and adjusted by the driver.
Actuation of the parking brake by independent energy may be effected in a variety of different manners. Such actuation appropriately makes use of electrically drivable energy sources (such as booster, hydraulic pump, plunger drive, accumulator with electric valves) which are already provided in the brake system. It is favorable that the independent energy source acts upon two hydraulic brake circuits, isolated from each other, in order to render possible an emergency parking brake operation in the event of failure of one brake circuit. Should both the operating brake pressure generator and the independent energy source act by way of a brake circuit upon the wheel brakes with locking device, upon failure of this brake circuit, the ABS/TCS valves of the other brake circuit can be used for an emergency parking brake function to enclose the brake fluid quantity.
Controlled delivery of the parking brake force in the case of an actuation by independent energy may occur in a favorable manner by way of the activity period of an electric switch. On the other hand, controlled brake force delivery is also possible by the direct digital or analog input of the desired braking pressure intensity.
In another preferred aspect of the present invention, the mechanical locking device has an emergency lever which permits deactivating the locking device by way of tool means. Appropriately, an emergency lever of this type is directly connected mechanically to the unilaterally acting catch mechanism of the wheel brake and thus provides emergency release of the brake.
In still another preferred aspect of the present invention, the clamped piston position is locked mechanically by means of an automatic, infinitely variable adjusting device. The mechanical locking mechanism is activated electromagnetically by an electric circuit which is operable independently by an electric switch. This permits using the parking brake in the regular operating brake pressure range which obviates the need for independent energy sources that require a pump or an accumulator, for example.
In a favorable embodiment of the present invention, the adjusting device includes an adjusting nut and an adjusting spindle with a motional thread which is particularly positioned in the interior of the brake piston. Expediently, the friction clutch is provided by the adjusting nut and a friction cone accommodated in the brake housing.
In further advantageous embodiments, the electromagnet is arranged in the brake housing, and the armature of the electromagnet is rigidly connected to the adjusting nut.
Further, it has proved to be suitable that the winding of the electromagnet is incorporated in a cylindrical extension of the brake housing cover and the friction cone is provided on the cylindrical extension of the brake housing cover.
Advantageously, the adjusting nut has two axial bearings for positioning. One of the bearings is supported on the brake housing by way of a spring.
To prevent release of the parking brake by inadvertent depression of the brake pedal, an electric relay is disclosed in preferred embodiments. When the electromagnet is activated, the relay is operated so that the electromagnet is closed upon actuation of the brake light switch. Or, when the electromagnet is activated, the relay is operated so that actuation of the brake light switch causes energization of a normally open valve which isolates the parking brake from the hydraulic pressure supply. Another embodiment provides for this purpose for an alternating switch which, in its on-condition, energizes the electromagnet/the normally open valve when the brake light switch is operated.